In view of handling, health and safety of workers, air pollution and the like, it is desired that the photosensitive resin plate which is employed for flexographic printing can be developed with water rather than organic solvent. It is also required that the printing plate having a relief image after exposure and development has a chemical resistance against aqueous ink or alcohol-based ink which is usually used in flexographic printing. It is further desired that the physical and printing properties of the printing plate are stable and do not change between before and after printing.
Japanese Kokoku Publication 35481/1978 discloses a liquid type photosensitive resin composition for flexographic printing, which mainly contains a liquid unsaturated polyester. However, it is difficult that a resin composition layer on a substrate is formed with accurate thickness, because the composition is liquid. Also, for developing this composition, a specific air knife should be required.
A solid type photosensitive resin composition for flexographic printing is proposed in U.S. Pat. No. 4,369,246, but this is developed with organic solvent which arises the problems caused by the organic solvent, as mentioned above.
Water developable photosensitive resin compositions for flexographic printing are also proposed in U.S. Pat. Nos. 2,893,868, 4,275,142 and Japanese Kokai (unexamined) Publication 22339/1986. However, the compositions of the both U.S. patents do not have sufficient water developability for industrial scale printing and the resin plates obtained therefrom have poor water resistance. The printing plate obtained by the Japanese specification also has poor mechanical strength.
In order to improve water developability, U.S. Pat. No. 4,177,074 and Japanese Kokoku (examined) Publication 45416/1985 propose a combination of a high molecular weight conjugated diolefinic hydrocarbon and a low molecular weight conjugated diolefinic hydrocarbon, but mechanical strength is not improved.